The present invention relates in general to the processing of semiconductor substrates, and more particularly, to improved systems and methods for epitaxially depositing films onto a semiconductor substrate.
Processes utilizing epitaxial deposition generally involve the growth of one or more layers or films onto a semiconductor substrate. The growth of these layers is carefully controlled by the underlying processes and equipment to produce layers having the desired physical, electrical and mechanical characteristics. These characteristics typically include, for example, the growth rate and thickness of the epitaxial layer, resistivity, dopant concentration, doping transition width, defect density, level of metal and particle contamination, and slip. Because these characteristics are highly dependent upon the operating conditions (e.g., temperature, gas flow rate and concentration of process gases) under which the epitaxial layers are grown, the underlying processes and equipment must maintain precise control over these operating conditions in order to produce epitaxial layers having uniform characteristics over the entire surface of the semiconductor substrate. Maintaining the required level of control, however, has proven increasingly difficult to achieve due to the recent transition from 200 mm to 300 mm fabrication processes, the tighter process specifications imposed by many semiconductor manufacturers and the higher throughput requirements (the number of substrates processed per unit of time) necessary for cost-effective manufacturing. In light of these increased processing demands, maintaining the necessary level of control while simultaneously satisfying throughput requirements, has proven increasingly difficult to achieve using conventional epitaxial deposition approaches.
Referring to FIG. 1, an exemplary reactor for performing epitaxial deposition in accordance with an existing approach is illustrated generally at 100. The exemplary reactor consists of a quartz bell-shaped jar 101 which encloses a semiconductor substrate 102 and isolates the substrate 102 from outside contaminants. The bell-shaped jar 101 also encloses a susceptor 103 which is used to support and rotate the semiconductor substrate 102 during processing. The process gases used to deposit epitaxial layers are injected into the reactor through a gas inlet port 104 and are exhausted out of the reactor through an exhaust port 110 located at the opposite end of the reactor. In order to heat both the semiconductor substrate 102 and process gases to operating temperature, a number of quartz halogen lamps 112 are positioned around the upper portion of the bell-shaped jar 101 to radiate energy into the reactor through the transparent walls of the bell-shaped jar 101. An optical pyrometer 113 located above a small window 114 in the reactor wall detects the temperature of the reactor. The optical pyrometer 113 relays the temperature measurements to appropriate lamp control circuitry (not shown) which then increases or decreases the output of the halogen lamps 112 in response to the detected temperature of the reactor.
In operation, the exemplary reactor of FIG. 1 deposits an epitaxial layer on the semiconductor substrate 102 by injecting process gases into the bell-shaped jar 101 via the gas inlet port 104. These process gases typically include a silicon source gas, such as silicon tetrachloride (SiCl4), trichlorosilane (SiHCl3), and dichlorosilane (SiH2Cl2), and a carrier gas, such as hydrogen. The process gases may also include n-type dopants or p-type dopants, which may be provided by precursor gases such as arsine (AsH3), and phophine (PH3) or diborane (B2H6). The gas inlet port 104 directs a gas stream 105 of the process gases horizontally toward the semiconductor substrate 102. As the gas stream 105 approaches and passes over the semiconductor substrate 102, the relatively large volume of the bell-shaped jar 101 causes the gas stream 105 to split into a laminar flow 106, which flows across the surface of the substrate 102, and the naturally circulating flow 107 which fills the upper portion of the bell-shaped jar 101. The flow can be laminar or turbulent, depending on the temperature gradient and characteristic length (from susceptor surface to top surface of chamber enclosure). A boundary layer 108 is also produced above the surface of the semiconductor substrate 102 due to the velocity gradient between the laminar flow 106 and the relatively stationary (although rotating) semiconductor substrate 102.
As the laminar flow passes over the semiconductor substrate 102, some of reactants diffuse through the boundary layer 108 to adsorb on the surface of semiconductor substrate 102. Once adsorbed, the reactants 115 undergo surface diffusion to find an appropriate lattice site 116 on the growing single crystal film. This surface diffusion step requires energy, and is an important factor that determines the quality of the resulting epitaxial layer. If the surface energy is insufficient for a reactant to become accommodated at a lattice site before additional atoms have accumulated over it, undesirable defects in the crystal lattice will occur. The reactants 115 also react chemically with the surface of the semiconductor substrate 102 to form by-products 117, which desorb from the surface and diffuse through the boundary layer 108 back into the laminar flow 106 and are then removed from the reactor through the exhaust port 110.
Conventional epitaxial reactors, such as the reactor illustrated in FIG. 1, suffer from several deficiencies which may prevent these reactors from providing an effective and cost-efficient solution for various applications. One problem is that the bell-shaped jar 101 has a relatively large volume which inhibits the ability of the reactor to precisely control the processing temperature of the semiconductor substrate 102 and the process gases. As mentioned previously, the operating temperature of the semiconductor substrate 102 and process gases is a critical factor in achieving the desired physical, electrical and mechanical characteristics. If the temperature is too low, the reactants will have insufficient energy to become accommodated at an appropriate lattice site, which may result in an increase in the defect density within the crystal lattice. If the temperature is too high, in the case of SiH4 or Si2H6, silicon molecules will bond together to form silicon aggregates in the vapor phase. These silicon aggregates can then fall to the surface of the substrate 102 and interfere with single crystal growth. Although it would be desirable to reduce the volume of the bell-shaped jar 101, the ability to do so may be limited by structural integrity constraints of the reactor.
Another problem relates to the ability of existing reactors to control the flow and concentration of process gases over the surface of the semiconductor substrate 102. As the laminar flow 106 of FIG. 1 passes over the semiconductor substrate 102, reactants within the process gases will be steadily depleted so that the laminar flow 106 will have a lower concentration of reactants near the exhaust port 110 than near the gas inlet port 104. Although the affects of depleted reactant concentration can be reduced for the perimeter of the semiconductor substrate 102 by rotating the semiconductor substrate 102 during processing, the depleted reactant concentration over the inner portion of the semiconductor substrate are not adequately addressed by the gas flow system of FIG. 1. Consequently, the semiconductor substrate 102 will have a greater thickness and a lower resistivity around the peripheral portion of the semiconductor substrate 102 than the inner portion of the semiconductor substrate 102.
Yet another problem involves the potential for build-up of a low quality silicon film on the walls of the bell-shaped jar 101. Designers of so-called “cold-wall” reactors typically expend substantial effort to maintain the temperature of reactant gases high enough for reaction to occur, and, simultaneously maintain the temperature of bell-shaped jar 101 low enough to avoid the deposition of an amorphous, low quality silicon film on its walls. If the walls get too hot, however, silicon molecules will adhere not only to the semiconductor substrate 102, but also form a thin film on the quartz walls of the reactor. This undesired, low quality film can tint the bell-shaped jar 101 and cause a variety of problems. Because the amorphous silicon adheres poorly to the quartz bell-shaped jar 101, and because there is mismatch in thermal expansion coefficients of silicon and quartz, the silicon has a tendency to flake off the quartz walls when the reactor is cooled. The amorphous silicon can also flake off the quartz walls during processing and fall onto the semiconductor substrate 102 thereby producing particulate contamination. The tinted bell-shaped jar 101 also reduces the amount of energy that can pass through it from the halogen lamps 112. Additionally, amorphous silicon may coat window 114, which causes optical pyrometer 113 to receive less light energy than it otherwise would have. The optical pyrometer 113 will then erroneously detect that the wafer temperature is cooler than it should be, and will instruct the halogen lamps 112 to deliver more energy, leading to even more unwanted deposition on the bell-shaped jar 112. As a result, it is frequently necessary to etch these deposits off of the bell-shaped jar with HCl after each semiconductor substrate is processed, which decreases the throughput of the reactor.
Therefore, in light of the deficiencies of the prior art and the increasing importance of epitaxial deposition in a variety of integrated circuit technologies, such as bipolar junction transistor (BJT) and complimentary metal oxide semiconductor (CMOS) technologies, there is a need for improved systems and methods for depositing epitaxial layers on a semiconductor substrate.